Their forces are specialized in very high level troops, which makes it difficult to fight them. Usually what I do is leave a line of ranged troops behind infantry (ranks) in the back and charge in by myself with a few cavalry, I focus their ranged troops first. After their ranged troops and cavalry are dead, the infantry are easy to deal with if you're capable enough to hit and run. Alternatively...Cannons XD

The Key to winning any battle and defeating any army is to have a heavy armored infantry with short swords, shields and armed with pistols..
They will kill most of the enemy troops while charging/holding and when engaged in melee the enemy will already have been damaged or killed by a hail of innacurate but deadly pistol fire.
Its the ultimate unite. Make them look like romans with guns and its gg for the enemy.

I've also been trying custom troops armed with pistols, board shields and spears XD Because they usually run in, they sometimes shoot enemies right in the face with the pistol before switching to melee

I dont really have a issue fighting the mithridians. Let their lancer charge into your line (Do very tight formation to catch them in) Wipe the floor with their Crossbow men by engaging in what i call a scraping manuever with your cav (when you lightly scrape by the enemy line killing the more annoying) And then pull back and let your cannons and Crossbowmen pick them off bit by bit. when they begin fighting your infantry,do a hammer and anvil. 70% of the time it works,all the time